A DMM is an electronic device used to display and/or analyze electronic signal measurements in digital numeric and/or graphic form. In its simplest form, a DMM provides an electronic representation of instantaneous signal voltage, current, and/or resistance. Optionally, a DMM may display its readings as a bar graph or in some other form that provides a visual representation of magnitude.
A typical prior art digital multimeter is illustrated in FIG. 1. Referring to FIG. 1, the DMM includes a digital display 10 on which numeric values of signal readings are displayed. Wire probes or leads are connected to the DMM through various input terminals. A typical DMM includes a voltage and resistance input terminal 12 that is used to read signal voltage and/or wire resistance, a current or amperage input terminal 14 that is used for current measurements, and a common terminal 16 that is used for both voltage/resistance and current measurements. Often, two current input terminals are provided so that a first current input terminal 14 is available for small current measurements and a second current input terminal 18 is available for large current measurements. To measure a signal voltage, current, or a resistance, the user places leads in the common terminal 16 and either the voltage/resistance terminal 12 or one of the current terminals 14 and 18. The two leads are then placed across the item for which the measurement is desired and the measurement is displayed on the display 10. A function/range switch 20 is also typically provided to allow the user to choose the measurement to be displayed, (i.e., voltage, resistance, or current) and select a range of units for the display.
Although prior DMMs are typically hand-held devices, prior DMMs have been limited to stand-alone units. In areas with limited space such as service garages, it is desirable to have a portable and small device that can serve multiple functions. In addition, users who move from place to place find it undesirable to carry, store, transport, and care for multiple electronic devices. While the prior art has taught that a DMM may be provided with other functionality, such as that of an oscilloscope as described in U.S. Pat. No. 4,034,291, to Allen et al., to date general electronic devices have not been provided with add-on DMM functionality.
Accordingly, it is desirable to provide an improved adapter device that enables a portable electronic device to function as a DMM.